1. Field of the Invention
The present invention relates to a coating film-forming method, particularly to a coating film-forming method, which comprises coating a cationic electrodeposition coating composition containing a specified base resin and a specified curing agent consisting of a blocked polyisocyanate curing agent onto a substrate and which is capable of obtaining a coated product having good properties in throwing power, electrodeposition suitability for an anti-corrosive steel plate or coating properties for a galvanized sheet, and anti-corrosive properties.
2. Description of Background Art
The cationic electrodeposition coating composition has good coating workability and is capable of forming a coating film showing high anti-corrosive properties, resulting in being widely used as a primer coating composition coated on an electrically conductive metal product such as an automobile body and the like, in which above properties are required. A many folds-structure of reinforcing materials for the purpose of increasing strength of the automobile body from the standpoint of improving collision safety in recent years makes difficult to flow through and may reduce a current density, resulting in making difficult a deposition of a coating film and in reducing anti-corrosive properties due to failure of coating. For the purpose of solving the above problem, forming a thick film on an inner plate area by raising a coating voltage is proposed.
The galvanized sheet is widely used as an anti-corrosive steel sheet from the standpoint of a low cost of the automobile body. On the other hand, an electrodeposition coating in a short period of time of 90 to 150 seconds is demanded from the standpoints of a saving of energy, saving of space and improvement in productivity in a coating line.
In view of the above problems, a high voltage electrodeposition coating was forced for the purpose of obtaining a high throwing power on the surface of a pouch area or inner plate, resulting in producing such problems that sparks generated during the electrodeposition coating of the galvanized sheet on the surface of an outer plate of the substrate may remain as pinholes after heat-curing of the coating film, resulting in reducing finished properties.
Japanese Patent Application Laid-Open No. 2001-19878 discloses a method of preparing a cationic electrodeposition coating composition capable of controlling development of pinholes due to gas generation on the galvanized sheet and obtaining a coating film showing high throwing power, and further discloses an electrodeposition coating film-forming method, which method comprises subjecting an electrodeposition coating composition controlled under such conditions that a minimum film-forming temperature of the cationic electrodeposition coating composition is in the range of ±5° C. of a predetermined electrodeposition coating temperature and an electrical conductivity on coating is in the range of 1000 to 1500 μS/cm to an electrodeposition coating at the predetermined electrodeposition coating temperature.
Japanese Patent Application Laid-Open No. 2002-275690 discloses a coating film-forming method, which method comprises coating a cationic electrodeposition coating composition under an effective voltage (V) of 230 V or less, the cationic electrodeposition coating composition having a uniform coating properties capable of ensuring an inner film thickness (μm) without increasing an outer plate film thickness and showing good finished properties of the galvanized sheet, and having a polarization resistance (a) per unit film thickness in the range of 120 to 300 kΩ·cm2/μm and a coating composition deposition amount (b) per unit electrical quantity in the range of 50 to 150 mg/C in the cationic electrodeposition coating.
Japanese Patent Application Laid-Open No. 2002-60680 discloses a cationic electrodeposition coating composition showing improved properties in throwing power and resistance to pinhole development due to gas generation and further discloses the cationic electrodeposition coating composition containing an amine-modified epoxy resin and a blocked polyisocyanate curing agent as a resin component and containing an organic acid and a metal salt of the organic acid as the neutralizing agent under controlling a total amount of the organic acid and an equivalent ratio of the organic acid to the metal salt of the organic acid.
As above described, the cationic electrodeposition coating composition having both throwing power and electrodeposition coating suitability for the galvanized sheet is known in the art, but any cationic electrodeposition coating composition having throwing power, electrodeposition coating suitability for the galvanized sheet and good anti-corrosive properties in a short period of time of 90 to 150 seconds as an energizing time is not known in the art.